The project is aimed at elucidating the fundamental characteristics and mechanisms of human color vision and defects of human color vision. The exact ways in which properties of the cones themselves as compared to neural factors contribute to the mechanisms of human color vision are as yet not completely specified. Nor is it fully known how various diseases of the eye or the aging process may affect the cone photoreceptors or the neural connections in the color pathway. the project combines experimental and modeling approaches to these long-term objectives. The experimental approach is by means of psychophysical methods applied to human observers. One major current goal of the project is to determine the relative numbers and distribution of the cone types in the human retina. It is shown that the detection of test lights of small dimension can be related to the number of cones contributing to detection. The different cone types can be selectively isolated by appropriate choices of combinations of test and background lights. These measurements can be evaluated to obtain estimates of the relative numbers and distributions of the different cone types in the human retina. A second goal of the project is to use this information to formulate and evaluate more realistic and quantitative models of human color vision and human color vision deficiencies. These models can be more stringently tested, since they can be constrained by the knowledge of the relative numbers of different cone types.